536 Proceedings of the Uoyal Society 
substance. The gas between these and the manometric mercury, 
and the mercury serve merely the purpose of transmitting the steam- 
pressures, and measuring the difference 
between them. 
At 12J° C., the sensibility of the 
instrument, as we see by Regnault’s 
tables of sulphurous acid steam-pres- 
sures, quoted in the article “ Heat,” of 
the eleventh volume of the j Encyclo- 
pedia Britannica , is 7 centimetres 
difference of mercury levels, to 1° 
difference of temperature (that is 
temperatures 12°, 13°) at T, T' re- 
spectively. 
At 22J° the sensibility similarly reckoned, is 12 -2 cms. to 1°. 
Note on Steam-Pressure Thermometers. 
By Sir William Thomson. 
If the bore of the vertical tube is less than three or four milli- 
metres, there ought to be an enlargement at its upper end, or else 
there should not be quite enough of the liquid to fill the tall mano- 
metric tube ; otherwise, if in the use of the instrument the liquid 
is pressed up to the top of the vertical tube, it is impossible to get 
it down again except by the tedious operation of distilling the 
whole liquid from the tube into the bulb, by applying heat by 
means of a spirit-lamp or a large vessel of hot water to the mano- 
metric tube, which, to facilitate this operation, may be held in- 
clined with the closed end down. An instrument like that shown 
in fig. 1 of my former communication (March 1, 1880), with vertical 
tube of the diameter (2 or 3 millimetres) there stated is subject 
to this inconvenience, although, in my first attempts to realise the 
instrument, imperfect removal of air from the water and steam in 
the enclosed space prevented me from experiencing it. The diffi- 
culty, of course, might have been foreseen ; but I did not think it 
would have been so great as I now find it to be with an instrument 
constructed exactly according to fig. 1 of my former communication, 
with air very perfectly removed from the enclosure by a proper pro- 
cess of boiling before sealing the instrument. 
